1. Field of the Invention
The present invention relates to a video signal detection circuit and a video signal processing device, and, more specifically, relates to a video signal detection circuit for PLL synchronous detection which suppresses leakage onto a picture plane of so called C/S beat signals representing beat components of color sub-carrier waves and sound carrier waves in an intermediate frequency amplifier circuit (IF circuit) of a video signal processing device, such as a TV receiver or a VTR.
2. Background Art
In a video signal processing device such as a TV receiver or a VTR, intermediate frequency signals (IF signals) which are transmitted from a tuner in response to channel selection are normally received by a video signal detection circuit by a SAW filter (a comb filter). In this device, there are two methods for separating video signals from the sound signals. In one method, respective SAW filters for the video relating signals and for the sound relating signals are provided and these signals are separated at the IF stage, and in the other method the video relating signals and the sound relating signals are received by a common single SAW filter and the sound signal components are separated by a band pass filter (BPF) and transmitted to a sound detection circuit after video signal detection.
On the other hand, at the detection output of the video signal detection circuit other than luminance signals and chroma signals such as sound carrier wave signals of 4.5 MHz and C/S beat signals representing the beat of the sound signals and the chroma signals as explained above, for example a beat component normally of 920 kHz in an NTSC system, are generated. Since reproduced picture images are likely to suffer from video disturbance by the C/S beat signals, the sound relating signals are normally attenuated by about several tens of dB with respect to the video relating signals so as to suppress the C/S beats. In particular, since the frequency of the voice sound carrier waves of 4.5 MHz is high, these components are removed after the video signal detection with the provision of a trapping circuit.
In the former method in which the two separate SAW filters are provided, since the sound relating signals and the video relating signals are separated from each other, an advantage that the influence of the C/S beat signals is suppressed accordingly is obtained; however, the number of circuits which are to be formed into ICs is increased correspondingly to thereby increase the cost thereof. On the other hand, in the latter method in which the single common SAW filter is provided, the circuit scale thereof is reduced to thereby realize a system of a low cost, however, since sound signals flow through the video signal detection circuit, C/S beat signals are generated and the generated C/S beat signals appear on the picture plane which poses a problem of reducing the picture quality. In particular, in a case of a video signal detection circuit for a PLL synchronous system, since phase detection outputs are returned to a PLL, the circuit is more likely affected by the C/S beat signals in comparison with other detection systems.